Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae

Abstract

Context: .ISOPHOT multi-aperture photometry is an efficient method to resolve compact sources or to detect extended emission down to relatively faint levels with single detectors in the wavelength range 3 to 100 μm.
Aims: .Using ISOPHOT multi-aperture photometry and complementary ISO spectra and IR spectral energy distributions we discuss the nature of the extended IR emission of the two PNe NGC 6543 and NGC 7008.
Methods: .In the on-line appendix we describe the data reduction, calibration and interpretation methods based on a simultaneous determination of the IR source and background contributions from the on-source multi-aperture sequences. Normalized profiles enable direct comparison with point source and flat-sky references. Modelling the intensity distribution offers a quantitative method to assess source extent and angular scales of the main structures and is helpful in reconstructing the total source flux, if the source extends beyond a radius of 1 arcmin. The photometric calibration is described and typical accuracies are derived. General uncertainty, quality and reliability issues are addressed, too. Transient fitting to non-stabilised signal time series, by means of combinations of exponential functions with different time constants, improves the actual average signals and reduces their uncertainty.
Results: .The emission of NGC 6543 in the 3.6 μm band coincides with the core region of the optical nebula and is homogeneously distributed. It is comprised of 65% continuum and 35% atomic hydrogen line emission. In the 12 μm band a resolved but compact double source is surrounded by a fainter ring structure with all emission confined to the optical core region. Strong line emission of [ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes this spatial profile. The unresolved 60 μm emission originates from dust. It is described by a modified (emissivity index β = 1.5) blackbody with a temperature of 85 K, suggesting that warm dust with a mass of 6.4 × 10^-4 M_⊙ is mixed with the ionised gas. The gas-to-dust mass ratio is about 220. The 25 μm emission of NGC 7008 is characterised by a FWHM of about 50´´ with an additional spot-like or ring-like enhancement at the bright rim of the optical nebula. The 60 μm emission exhibits a similar shape, but is about twice as extended. Analysis of the spectral energy distribution suggests that the 25 μm emission is associated with 120 K warm dust, while the 60 μm emission is dominated by a second dust component with 55 K. The dust mass associated with this latter component amounts to 1.2 × 10^-3 M_⊙, significantly higher than previously derived. The gas-to-dust mass ratio is 59 which, compared to the average value of 160 for the Milky Way, hints at dust enrichment by this object.